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I am looking at reference schematic for Mic Preamplifier. I have posted a part of schematic here:Schematic

So the net VDD_MIC is used to provide a DC offset to signals SPKR_MIC_P, SPKR_MIC_N. I wonder whats the use of C483 here? For DC it's open however for AC R494 and C483 will act as a high pass filter, this will increase the coupling between the SPKR_MIC_P and SPKR_MIC_N.

What's the rationale behind doing this?

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Looks like it is just decoupling the rails from the bias. Basically forming two sub-rails a 1K drop from VDD_Mic and Ground.

If you redraw it this way it is a tad clearer.

schematic

simulate this circuit – Schematic created using CircuitLab

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